Wireless local area networks (WLANs) typically transmit via radio or infrared frequencies to connect data devices. In a WLAN, the wireless communication devices are often mobile, moving around more or less freely within the networked area. WLANs combine with infrastructure networks systems that can be connected to the Internet, thereby providing communication over long distances.
WLANs link portable and wireless computer devices, also called mobile stations or terminals, to a wired LAN via a plurality of fixed access points (APs), also called base stations. Allowing WLAN devices to communicate with the infrastructure network, access points provide for wireless communications within respective cells and are typically spaced throughout a designated networked area. The access points facilitate communications between a networked set of 802.11-compliant devices called a basic service set (BSS), as well as communications with other BSSs and wired devices in or connected to wired infrastructure network systems.
WLANs have been used in proprietary business applications such as order entry, shipping, receiving, package tracking, inventory, price-markdown verification, and portable point of sale. Such systems may have an operator carrying a handheld computer device that communicates with a server via one or more access points such as a wireless bridge or router, each access point interacting with the server to create a wireless cell.
The most common WLAN technologies are described in the Institute of Electrical and Electronics Engineer's IEEE 802.11 family of industry specifications, which include two physical-layer standards: 802.11b operating at 2.4 GHz and delivering up to 11 Mbps at 250 feet maximum; and 802.11a operating at 5 GHz and delivering up to 54 Mbps at 150 feet maximum. A third standard, 802.11g, provides the speeds of 802.11a at the distances of 802.11b. IEEE 802.11 specifies Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) for devices operating within an 802.11 wireless network. Informative material may be found in IEEE Std. 802.11-1999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, reference number ISO/IEC 8802-11:1999, ANSI/IEEE Std. 802.11, 1999 edition, 1999.
When a wireless devices moves around a WLAN, it may need to change its association from one access point to another if the reception level of the associated access point becomes too low. The procedure known as roaming allows a WLAN device to switch between access points, a change that is generally based on the relative reception levels of the access points involved. Roaming procedures may be based on selected configuration settings for the access points (APs) such as density levels of cell sizes that influence their defer, carrier-detect, and cell-search behaviors.
Within the wireless networks, wireless communications are generally managed according to an operating protocol that requires ongoing wireless activity to monitor the roaming of WLAN devices and to synchronize radio timing between these portable devices and access points. This ongoing activity contributes to the draining of power from battery-powered WLAN devices. Synchronization of radio timing becomes especially critical in the management of wireless communications, and more efficient scheduling of future coordinated activities provides better power-saving strategies.
Before a WLAN device can communicate with other devices in a given WLAN, it must first locate access points. The medium access control (MAC) layer-2 protocol of the IEEE 802.11 manages, coordinates and maintains communications, traffic, and data distribution in wireless networks that have fixed access points or in ad hoc networks. The IEEE 802.11 MAC protocol defines beacon frames sent at regular intervals, known as beacon intervals, for example, every 100 microseconds, by an access point that allow WLAN devices to monitor for the presence of an access point. Passive and active scanning techniques have been developed for WLAN devices to detect access points, although the 802.11 standard does not mandate particular methods for scanning.
Passive scanning allows the network interface card (NIC) of a WLAN device to find an IEEE 802.11 network by listening for traffic. As defined in 802.11, passive scanning involves a WLAN device listening to each frequency channel for no longer than a maximum duration defined by the ChannelTime parameter. In this passive mode, the wireless NIC listens for beacons from neighboring access points, while extracting information about the particular channel. Passive scanning expends time and battery power while listening for a beacon frame that may never occur or may be on an idle channel.
The ChannelTime is configured during the initialization stage of the WLAN device driver. To initiate a passive scan, the driver commands the firmware to perform a passive scan with a list of channels. The firmware sequences through the list of channels and sends any received frames to the driver. The amount of time spent on the channel is equal to the ChannelTime value. The driver is able to abort the passive scan when the desired beacon or probe response is received.
Active scanning, in contrast to passive scanning, requires the scanning wireless NIC to transmit requests and receive responses from other 802.11 wireless NICs and access points. Active scanning allows the mobile wireless NIC to interact with another wireless NIC or access point based on probe requests and probe responses.
The active scanning of the IEEE 802.11 MAC uses a set of management frames including probe request frames that are sent by a WLAN device and are followed by probe response frames sent by an available access point. In this way, a WLAN device may scan actively to locate an access point operating on a certain channel frequency and the access point can indicate to the WLAN device what parameter settings it is using.
In an active scan, the WLAN device transmits a probe request frame, and if there is a network on the same channel that matches the service set identity (SSID) in the probe request frame, an access point in that network will respond by sending a probe response frame to the WLAN device. The probe response includes information the WLAN device uses to access a description of the network. The WLAN device processes the beacon frames and any additional probe responses that it may receive.
Once the various responses are processed or it has been determined that no response has been received within a prescribed time, a WLAN device may continue to scan on another radio channel. At the end of the scanning process, the WLAN device has accumulated data about the networks in its vicinity, and the device can determine which network to join. When compared to passive scanning, active scanning results in longer battery life for the WLAN device, but it also reduces network capacity.
After passive or active scanning, a WLAN device registers itself with the AP of the chosen network, synchronizes with the AP and, thereafter, transmits and receives data to and from the AP. According to the IEEE 802.11 standard, the registration includes an authentication whereby the AP identifies whether a WLAN device has the right or not to access the wireless network via a medium access control (MAC) layer. Generally, this authentication phase requires bi-directional authentication steps with the AP and WLAN device exchanging some packets, and optionally, may include additional steps of assertion of identity, challenge of assertion, and response to challenge. After authentication, the WLAN device establishes a connection link with the AP by sending an association request packet to the AP and waiting to receive a response frame from the AP that acknowledges the association. The WLAN device joins a basic service set (BSS) by setting its local hopping time and channel sequence according to the information contained in the AP beacon.
The AP is the timing master of the network, performing a TSF (timing synchronization function) to keep the timers for all WLAN devices synchronized within the same basic service set (BSS) of a larger network. The beacons that are broadcast at fixed time intervals by the AP contain copies of the TSF timer and hopping sequence to synchronize other WLAN devices in a BSS. When a timestamp of a device's TSF timer is different from the timestamp in the received beacon frame, the WLAN device resets its timestamp value to match the received timestamp value.
The total time that is consumed for devices using IEEE 802.11 WLAN and other wireless communication technologies to complete all the steps of scanning, authentication and association can vary greatly. Thus, improving the scanning process for wireless networks would help the establishment of a connection between devices and the communication within a network to become more predictable, as well as to become more power and time efficient, particularly for battery-powered IEEE 802.11 WLAN devices. More effective programming techniques for scanning would minimize the number of probe requests generated, the amount of time the receiver of the device is set to an on-state, and the number of times the firmware is interrupting a host controller for beacon processing. Thus, the improved scanning system would increase the battery life of a WLAN device, because the device would need less time to scan or monitor for beacon signals from a primary as well as neighboring access points. In addition, improvements of the scanning system for a WLAN network would benefit associated networks such as wide area networks (WAN), personal area networks (PAN), and controller area networks (CAN).